An image forming system may be provided that includes an image forming apparatus capable of forming an image on a recording sheet such as a copier or a printer, and a post-processing apparatus capable of applying post-processing to the recording sheet having the image formed thereon. For example, the post-processing apparatus may bind a plurality of recording sheets output from the image forming apparatus to generate a book document. Recently, the post-processing apparatus is provided with a wide variety of functions to create a book document, such as the function of saddle-stitching as described in Japanese Patent Application Registration No. 3875802 or paperback binding as described in Japanese Patent Application Publication No. 2008-132728.
In the image forming apparatus, a transfer device transfers the recording sheet having a toner image formed thereon through a fixing device. The fixing device fixes the toner image onto the recording sheet by heat and pressure. In order to sufficiently fix the toner image onto the recording sheet, a temperature of the recording sheet having the toner image thereon needs to be sufficiently increased so as to cause toner in the toner image to melt. This may be achieved by either increasing a fixing temperature of the fixing device or by reducing a transfer speed at which the recording sheet is transferred through the fixing device. However, a higher fixing temperature may result in degradation in image quality such that a desired level of gloss or color may not be obtained. Further, the reduced transfer speed of the recording sheet may reduce the overall processing speed to compete printing operation. For this reason, the fixing device fixes the toner image onto the recording sheet with the lowest fixing temperature that sufficiently fixes the toner image. Further, the transfer device transfers the recording sheet with the highest transfer speed that sufficiently fixes the toner image. This may sometimes cause the toner image to be partially removed especially when the recording sheet is folded by the post-processing apparatus. In case of creating a book document, the book document that is created may not be of good quality if the toner image is partially removed from the recording sheet, which is to be placed on the outer surface of the book document.
In view of the above, Japanese Patent Application Publication No. 2007-084324 discloses a sheet processor provided with a heating roller pair that heats the bound part of a bundle of sheets such that toner fixed onto the bound part is melted. While this technique is able to prevent toner from being partially removed from the recording sheet, the heating roller pair needs to be additionally installed onto the sheet processor, thus increasing a size of the sheet processor and increasing the overall manufacturing cost for the sheet processor.
There is a need for an apparatus, method, or system capable of preventing toner in the fixed toner image from being partially removed from the recording sheet even when the recording sheet having the fixed toner image is folded to create a book document, while keeping down the overall size and the manufacturing cost of the post-processing apparatus or the image forming apparatus.